Method of stacking a plurality of dies to form a stacked semiconductor device, and stacked semiconductor device

ABSTRACT

A method of stacking a plurality of first dies to a respective plurality of second dies, each one of the first dies having a surface including a surface coupling region which is substantially flat, each one of the second dies having a respective surface including a respective surface coupling region which is substantially flat, the method comprising the steps of: forming, by means of a screen printing technique, an adhesive layer on the first dies at the respective surface coupling regions; and arranging the surface coupling region of each second die in direct physical contact with a respective adhesive layer of a respective first die among said plurality of first dies.

BACKGROUND

1. Technical Field

The present disclosure relates to a method of stacking a plurality ofdies to form a stacked semiconductor device or system, and to thestacked semiconductor device or system.

2. Description of the Related Art

Currently assembly of a plurality of dies, where one of them has athickness equal to, or lower than, 50 μm, includes a specific techniquein order not to damage the thin die and allow die attach film (DAF) tapeseparation. Due to the wafer sawing process, such a thin wafer issingulated by means of a “Dicing Before Grinding” (DBG) process. Thisleaves singulated dies face down on a grinding tape. The same dies,then, are further laminated onto either a mounting tape or onto a DAFtape and a mounting tape to be processed at die attach.

In case of using a mounting tape only, a DAF tape is pre-applied, with atape cut-and-place process, to the thicker die. The same process is noteasily adaptable to the thinner die.

In the case where the singulated die is laminated onto a DAF tape, thelatter is in some way separated at the sawing kerfs. Two methodsavailable are either laser DAF cutting or low temperature expansion ofthe DAF tape which causes a rupture of the DAF tape along the existingkerf lines.

The process of tape cut-and-place utilizes additional hardware optionson the die attach machines and is limited in application to large dies,due to economic reasons and availability of tape reels of the requiredwidth, as well as the manipulation of reduced tape sizes.

The method of DAF laser cutting is applicable to any die size. Thelimitations of this method are mainly due to high costs and low processthroughput.

The method of DAF expansion at low temperatures utilizes dedicatedequipment. To date this method is limited to large die sizes and is notapplicable to die sizes less than 3×3 mm. Moreover the process is notisotropic and the DAF tape breakages are inconsistent across the waferaxes.

There is an ever present need to reduce package dimensions particularlythe package height by the consumer market. Die thickness reduction is acritical element to achieve. A grinding process to achieve thin dies asfar as 20 μm is available. Accordingly, there is a desire for achievinga cost effective way to perform die attach for such thin dies. Inparticular, there is a desire to achieve a cost effective and reliablemethod to stack a thin die on top of a MEMS sensor.

BRIEF SUMMARY

One or more embodiments of the present disclosure is to provide a methodof stacking a plurality of dies to form a stacked semiconductor device,and a stacked semiconductor device. One embodiment or more embodimentsmay achieve a cost effective and reliable method to stack a thin ASICdie (e.g., 50 μm or less) on top of a MEMS sensor.

One embodiment is directed to a method of stacking a plurality of firstdies to a respective plurality of second dies. Each one of the firstdies has a first surface coupling region that is substantially flat, andeach one of the second dies having a second surface coupling region thatis substantially flat. The method includes forming, using a screenprinting technique, an adhesive layer on the first dies at the firstsurface coupling regions. The method further includes arranging thesecond surface coupling region of each second die in direct physicalcontact with a respective adhesive layer of a respective first die amongsaid plurality of first dies.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

For a better understanding of the disclosure, preferred embodimentsthereof are now described, purely by way of non-limiting example andwith reference to the annexed drawings, wherein:

FIG. 1 shows a cross lateral section of a device housing a MEMSstructure, according to a known embodiment;

FIG. 2 shows the device of FIG. 1 coupled to a screen mask for screenprinting, according to a manufacturing step according to an embodimentof the present disclosure;

FIG. 3 shows a screen mask which can be used during the manufacturingstep of FIG. 2;

FIGS. 4-7 shows further manufacturing step carried out on the device ofFIG. 1, according to an embodiment of the present disclosure; and

FIGS. 8-14 shows manufacturing steps carried out on a plurality ofdevices of the type shown in FIG. 1, according to a further embodimentof the present disclosure.

DETAILED DESCRIPTION

Referring to FIG. 1, a device 4 includes a substrate 6, for example ofsemiconductor material, in particular silicon or germanium, having a topsurface 6 a opposed, along a direction Z, to a bottom surface 6 b. Thetop surface 6 a does not have bumps or protrusions or exposed activeparts. As better explained later on, the top surface 6 a may haveinstead one or more recesses. With the exception of the recesses, thus,the top surface 6 a is flat or substantially flat. In this context, asurface flat or substantially flat means a surface having a roughness ofsome tens of nanometers, e.g., in the range 10-100 nm, typically 65 nm.

The device 4 includes a MEMS structure 8 integrated in the substrate 6.Accordingly, in the following description, the device 4 is also referredto as MEMS device 4. Furthermore, the device 4 may also include afurther MEMS structure and/or an integrated circuit (not shown). Thesubstrate 6 may also include any plurality of layers formed thereon suchas epitaxial layer(s), insulating layer(s), dielectric layer(s),conductive layer(s), and/or other suitable materials.

The MEMS structure 8 includes, according to an embodiment, elementsforming a motion sensor (e.g., an inertial sensor such as a gyroscope oran accelerometer, an oscillator, and/or any other MEMS device which doesincludes elements exposed over the top surface 6 a). In this way, theMEMS structure 8 is effectively integrated within the substrate 6 anddoes not perturb the substantial flatness of the top surface 6 a.

As said, the MEMS device 4 further includes one or more recesses 10 (tworecesses are shown in FIG. 1), formed in the device substrate 6. Therecesses 10 are formed at the top surface 6 a, at peripheral regions ofthe MEMS device 4. The recesses 10 may be formed by a sequence ofphotolithographic and etching processes which are per se known,including wet etch and/or dry etching. The position, dimension and shapeof the recesses 10 may be determined by the layout design for the MEMSdevice 4. The recesses 10 house respective electrical connectingelements 12, in particular bond pads 12 which form bonding sites for theMEMS device 4. In other words, the bond pads 12 are electricalconductive elements that provide for I/O electrical interconnections toa further substrate (e.g., substrate of a package) and/or device.

According to an embodiment, recesses 10 have a depth t_(R), measuredalong the Z direction starting from the top surface 6 a, of at least 100μm, for example equal to approximately 130 μm.

The bond pad 12 housed by each recess 10 is made, for example, of metalsuch as aluminum, copper, gold, or any other suitable conductivematerials different from metals, e.g., doped polysilicon. According toan embodiment, each bond pad 12 has a thickness t_(P), measured alongthe Z direction, in the range of about 0.8 μm-1 μm. The bond pad 12 maybe an I/O bond pad providing connection to one or more elements orfeatures integrated within the substrate 6 (e.g., the MEMS structure 8and/or an integrated circuit), as shown in FIG. 1 by means of arrows 13.Furthermore, the bond pad 12 may be connected to a multi-levelinterconnect including a plurality of conductive lines and vias formedthrough the substrate 6 (not shown).

According to an aspect of the present disclosure, an adhesive layer, inparticular a paste layer, is applied on the top surface 6 a of thesubstrate 6 but not within the recesses 10. In this way, the top surface6 a effectively forms an interface for stacking a further die withoutcompromising the electrical properties of the bond pads 12 in therecesses 10. In particular, the paste used to form the paste layer is aB-stageable paste, more in particular a B-stageable epoxy resin, whichis known in the art. According to an embodiment, the paste used isABLECOAT® 8006NS. The term “B-staged”, or “B-stage”, or “B-stageable”,in reference to an epoxy resin, is commonly used to identify a singlecomponent epoxy system, using a latent, or low reactivity, curing agent.This product can be partially cured, as an initial stage, after beingapplied onto one substrate/surface. It can, at a later time, becompletely cured with a heat treatment.

Other pastes can be used, such as generally known die attach adhesives,or screen-printable adhesive with glue fillet control properties.

According to an embodiment of the present disclosure, after providingthe MEMS device 4 of FIG. 1 (or any other device having a substantiallyflat top surface), a screen mask 20, adapted for screen printing, isdisposed over the top surface 6 a of the substrate 6.

A portion of the mesh structure 21 of the screen mask 20 is shown, byway of example, in FIG. 3. The mesh structure 21 comprises a pluralityof stainless steel wires 21 a extending parallel to one another and aplurality of stainless steel wires 21 b extending parallel to oneanother and orthogonal to the stainless steel wires 21 a.

With reference to FIG. 3, the mesh count w and the wire diameter d isthe same for stainless steel wires 21 a and 21 b. In particular, meshcount w refers to the number of wires per inch contained in the meshstructure 21; the wire diameter d refers to the diameter of each wire 21a, 21 b which has been woven into the mesh structure 21.

According to an embodiment of the present disclosure, mesh count w isequal to 63 wires/inch, and the wire diameter d is equal to 36 μm.Accordingly, the mesh structure 21 has an open area A₀ (total open areain terms of mesh apertures across the entire surface of the meshstructure 21) of about 40%. Alternatively, mesh structures havingdifferent open area A₀ may be used.

Referring to FIG. 4, a screen-printing type paste 26, e.g., aB-stageable non-conductive epoxy resin, is set on the screen mask 20,and a screen printing step is carried out. The screen printing step iscarried out as it is commonly known in the art, i.e., by means of asqueegee 24 which is, for example, a flat, smooth rubber blade. Metalsqueegees may be used instead.

A fine alignment between the screen mask 20 and the regions over whichthe paste 26 is to be deposited may be eliminated. In particular, whenthe entire top surface 6 a has to be covered with an adhesive pastelayer, a screen mask 20 covering the entire top surface 6 a, or evenextending out of the borders of the top surface 6 a, may be used.Furthermore, as the screen mask 20 shows uniformly distributedapertures, no fine alignment is needed. Accordingly, the screen mask 20completely overlaps the top surface 6 a and the recesses 10.

In greater detail, after the paste 26 is deposited on the screen mask20, the squeegee 24 is disposed onto the screen mask 20, and moved ontothe screen mask 20, to print the paste 26. This is schematically shownin FIG. 5 by means of arrow 27. During this printing step, the squeegee24 moves and presses the paste 26 with an appropriate pressure in orderto allow the paste 26 to pass through the apertures of the screen mask20, thus reaching, and adhering to, the top surface 6 a of the substrate6. The Applicant has noted that, in the regions where the screen mask 20is not in direct contact with the top surface 6 a during printing (i.e.,at the recesses 10), the paste 26 does not leak out of the screen mask20 and, accordingly, no paste flows, or is deposited, within therecesses 10.

Referring to FIG. 6, the screen mask 20 is separated from the device 4,leaving a paste layer 30 on the top surface 6 a. The paste layer 30extends at regions of the top surface 6 a corresponding to regionspreviously covered by the screen mask 20 and which came into directcontact with the paste 26 during the step of FIG. 5; accordingly, thepaste layer 30 extends around the recesses 10 but not within therecesses 10. Moreover, even if the screen mask 20 exceeds the maximumdimensions of the device 4 along X and Y directions, paste 26 is notpresent around the device 4 (other than the stop surface 6 a) or alonglateral walls 6 c of the device 4.

After formation of paste layer 30, a first thermal process is carriedout, to partially cure the B-stageable paste 26. The thermal processcomprises placing the device 4 in a oven, rising the temperature toabout 100° C. with a temperature ramp of about 30 minutes, andcontinuing the curing step at a fixed temperature of about 100° C. forabout 60 minutes. Other thermal treatments are available, according tospecific paste used.

Referring to FIG. 7, a step of die attach is carried out. This step iscarried out according to a standard die-attach technique. A die 31 is,according to an embodiment, a die housing an ASIC circuit, which hasalready been processed under DBG (“Dice Before Grinding”) process, toachieve a thickness t_(ASIC) equal to, or below, 50 μm (e.g., in therange 20-50 μm). The ASIC die 31, after manufacturing process forforming ASIC circuitry and after grinding, is placed over the topsurface 6 a of the substrate 6 of the MEMS device 4, in direct contactwith the paste layer 30. The paste layer 30 thus forms an interfacebetween the thin ASIC die 31 and the MEMS device 4, guarantying adhesionamong them.

The maximum adhesion is achieved by a further step of thermal process tocure the B-stageable paste forming a fully-cured paste layer 30. Thisthermal process comprises placing the MEMS device 4 with ASIC die 31attached in an oven, rising the temperature to about 170° C. with atemperature ramp of about 30 minutes, and continuing the curing step ata fixed temperature of about 170° C. for about 60 minutes.

The above described process can be applied to a plurality of MEMSdevices 4, in a mass production facility.

FIG. 8 shows, in lateral view, the support substrate 35 over which aplurality of MEMS devices 4 of the type shown in FIG. 1 are attached.Each MEMS device 4 is coupled to the support substrate 35 through anadhesive layer 36 which is, for example, a layer of adhesive paste, ordie attach film. The support substrate 35 is, more in particular, astandard organic substrate, in particular an organic FR-4 substrate.

According to an embodiment, the arrangement of the plurality of MEMSdevices 4 over the support substrate 35 is an initial step for carryingout a packaging operation of the plurality of MEMS devices 4, and ismade according to the known art. According to this embodiment, thesupport substrate 35 moreover comprises a plurality of conductive (e.g.,metal) pads, arranged around each MEMS device 4, providing electricalconnections for the bonding pads 12. The conductive pads may includepads 33 a which, after packaging, remains internal to the package,configured to be electrically connected (e.g., through wire bonding) tothe pads 12; and pads 33 b which, after packaging, remains external tothe package, configured to form an electrical interface to access thebonding pads 12 of the respective MEMS device 4. Pads 33 a and 33 b areconnected to one another through a conductive path 33 c which may beformed integrated in the support substrate 35.

FIG. 9 is a top view of the support substrate 35 housing the pluralityof MEMS devices 4 according to FIG. 8. FIG. 8 is a cross section of FIG.9 taken along cut line VIII-VIII.

The plurality of MEMS devices 4 is arranged in a matrix form over thesupport substrate 35, and are aligned along rows (parallel to Xdirection) and columns (parallel to Y direction). Each device 4 isseparated, from an immediately successive device 4 arranged along a samerow, by a distance d_(R) equal to about 0.25 mm. However, the distanced_(R) may vary and may be in the range of 0.18-0.4 mm. Analogously, eachdevice 4 is separated, from an immediately successive device 4 arrangedalong a same column, of a distance d_(C) equal to about 0.25 mm.However, the distance d_(C) may vary and may be in the range of 0.18-0.4mm.

Referring to FIG. 10, a screen mask 37 is arranged over the plurality ofMEMS devices 4, in such a way that the screen mask 37 is in directcontact with the top surfaces 6 a of each MEMS device 4. Alternatively,at this step, the screen mask 37 does not need to be in direct contactwith the top surfaces 6 a of each MEMS device 4, but it is adapted tocontact directly the top surfaces 6 a of each device 4 during asubsequent screen printing step.

The screen mask 37 is analogous to the screen mask 20 previouslydescribed with reference, in particular to FIG. 3. In this case, thescreen mask 37 is shaped in such a way to completely overlap theplurality of MEMS devices 4 when placed in position according to FIG. 10(e.g., the screen mask 37 has, in a top view, the same shape as thesupport substrate 35). Then steps analogous to the steps previouslydescribed with reference to FIGS. 4 and 5 are carried out. A paste 26 isdeposited on the screen mask 37 and, by means of the squeegee 24, aportion of the paste 26 is transferred through the screen mask 37 to thetop surfaces 6 a of each MEMS device 4 where the screen mask 37 comes indirect contact with the top surfaces 6 a of the MEMS devices 4.

Referring to FIG. 11, the screen mask 37 is removed and the paste layer30 remains on the top surface 6 a of each MEMS device 4.

According to an embodiment of the present disclosure, the apertures ofthe screen mask 37 are not covered or obstructed at the regions of thescreen mask 37 which are aligned, when the screen mask 37 is set inposition over the devices 4, with the spacing existing between MEMSdevices 4. Accordingly, the screen mask 37 has a substantially uniformpattern over its entire extension.

As already said, the Applicant has noted that, where the screen mask 37is not in direct contact with the top surface 6 a of MEMS devices 4(i.e., in the regions between MEMS devices 4), the paste 26 does notleak out of the screen mask 37 and, accordingly, no paste 26 flowsthrough the mask screen 37 towards the support substrate 35 or betweenthe top surfaces 6 a of the MEMS devices 4.

After forming the paste layer 30, a step of die attach is carried out,as already disclosed with reference to FIG. 7. The die attach step isknown in the art. Referring to FIG. 12, the die attach process isperformed using a pick operation from a singulated wafer and a dieattach operation to attach the die onto another surface are carried out.In this case, since the adhesive has already been deposited, the die tobe attached (e.g., ASIC die) does not need any die attach film adheredto its back-side. In the case of a b-stage material, the heat blockshould be set at a temperature above a known temperature (in this caseit was 100° C.) to change the b-stage material consistency from solid toa soft tacky substance for attachment. In the case of adhesive, thebond-site temperature during die attach should be maintained at roomtemperature (which, however, may vary depending on the material selectedfor the application).

Thus, a plurality of stacked dies 40 is formed, each of the stacked dies40 including one MEMS device 4 and a respective ASIC die 31, coupledtogether by means of the paste layer 30.

A thermal process to fully cure the B-stageable paste, forming thefully-cured paste layer 30, is carried out for the plurality of stackeddies 40. A maximum adhesion is thus achieved. This thermal processcomprises placing the support substrate 35 with the stacked dies 40 inan oven, rising the temperature to about 170° C. with a temperature rampof about 30 minutes, and continuing the curing step at a fixedtemperature of about 170° C. for about 60 minutes. Referring to FIG. 13,a wire bonding step is carried out, connecting the bonds pad 12 of eachMEMS device 4 to respective pads of the support substrate 35 throughbonding wires 44.

A protective structure (or cap) 47 is then formed over and around eachstacked die 40, thus completing the manufacturing process for forming apackage 45.

Referring to FIG. 14, packages 45 are singulated into individual units49 (“dicing”). The dicing process can be accomplished by scribing andbreaking, by mechanical sawing or by laser cutting. During dicing, thesupport substrate is typically mounted on a dicing tape.

It is noted that the steps described with reference to FIG. 13 (wirebonding and formation of the cap) may be carried out after the dicingprocess.

The device, or system, thus formed, comprising a first die (e.g.,housing a MEMS sensor) coupled to a second die (e.g., housing an ASICcircuit), is adapted to carry out sensing operations to sense anexternal quantity, transduction of the sensed quantity, and signalelaboration. The device, or system, includes one among (but is notlimited to) an inertial sensor, such as an accelerometer or a gyroscope,magnetometer, pressure sensor, humidity sensor and any kind of sensingdevice in an enclosed cavity with flat top.

From an examination of the characteristics provided according to thepresent disclosure, further advantages emerge clearly.

In particular, according to the present disclosure, the adhesive layerbetween the MEMS sensor and the thin ASIC die can be deposited infeasible and economically advantageous way. Moreover, all the requiredmanufacturing steps for forming the adhesive layer are executed on the(thick) MEMS device. Furthermore, since according to the presentdisclosure there is no need to apply a DAF tape on the thin ASIC die,there is also no need for carrying out a DAF separation process afterthe ASIC dicing process.

Furthermore, process steps and the hardware used may be the sameirrespective of the size and shape of the MEMS devices, and irrespectiveof the particular layout of the top surface of the MEMS device (providedthat no protrusions exists on the top surface of the MEMS device).

The use of a B-stageable paste allows a cost reduction with respect to aDAF tape.

Finally, it is clear that modifications and variations may be made tothe embodiments described and illustrated herein, without therebydeparting from the scope of protection of the present disclosure.

In particular, even if the method according to the present disclosurehas been disclosed and illustrated with explicit reference to the stepsrequired for stacking two dies, it can be employed to stack any numberof dies one over the other. In this case, each die is physically coupledto another die through an adhesive paste layer formed by means of screenprinting technique, as described.

The various embodiments described above can be combined to providefurther embodiments. These and other changes can be made to theembodiments in light of the above-detailed description. In general, inthe following claims, the terms used should not be construed to limitthe claims to the specific embodiments disclosed in the specificationand the claims, but should be construed to include all possibleembodiments along with the full scope of equivalents to which suchclaims are entitled. Accordingly, the claims are not limited by thedisclosure.

1. A method comprising: stacking a plurality of first dies with arespective plurality of second dies, each one of the first dies having afirst surface coupling region that is substantially flat, each one ofthe second dies having a second surface coupling region that issubstantially flat, the stacking including: forming, using a screenprinting technique, a plurality of adhesive layers at the respectivefirst surface coupling regions of the first dice, respectively; andarranging the second surface coupling region of each second die indirect physical contact with a respective adhesive layer of theplurality of adhesive layers respectively, on said first dies.
 2. Themethod according to claim 1, further comprising: attaching saidplurality of first dies on a support substrate in a spaced apartarrangement with spacings between neighboring first dies; arranging ascreen mask on said plurality of first dies, the screen mask overlappingsaid first surface coupling regions of the first dies and said spacingsbetween neighboring first dies, the screen mask having a plurality ofuniformly distributed apertures, wherein forming, using the screenprinting technique, the adhesive layer includes: dispensing an adhesivepaste on the screen mask; and moving and pressing said adhesive paste onthe screen mask causing the adhesive paste to pass through the aperturesof the screen mask until the adhesive paste comes in direct contact withthe respective first surface coupling regions of the plurality of firstdies.
 3. The method according to claim 2, wherein at least one of thefirst dies has a recess proximate the first surface coupling region,wherein arranging the screen mask on said plurality of first diescomprises arranging the screen mask so that the screen mask overlapssaid recess.
 4. The method according to claim 2, wherein said screenmask comprises a mesh structure made of stainless steel wires.
 5. Themethod according to claim 2, wherein the support substrate is an organicsubstrate configured to form a support base of a package, the methodfurther comprising placing a cap over the support substrate covering andprotecting the first and second dies.
 6. The method according to claim1, wherein each one of the first dies houses an integrated MEMSstructure of a motion sensor, and each one of the second dies houses anintegrated ASIC circuit.
 7. The method according to claim 1, whereineach one of the second dies has a maximum thickness, measured along adirection orthogonal to said second surface coupling region of therespective second die, in a range between 20-50 μm.
 8. The methodaccording to claim 1, wherein forming, using the screen printingtechnique, the adhesive layer comprises forming an epoxy-basedB-stageable paste layer using the screen printing technique.
 9. Astacked semiconductor device, comprising: a first die including a firstsurface coupling region that is substantially flat; a second dieincluding a second surface coupling region that is substantially flat,the second die having a thickness, measured along a direction orthogonalto said second surface coupling region, that is between 20-50 μm; and anadhesive layer of a B-staged epoxy-based paste located between the firstand second coupling regions and coupling the first and second diestogether.
 10. The stacked semiconductor device according to claim 9,wherein the first die houses an integrated MEMS structure of a motionsensor and the second die houses an integrated ASIC circuit.
 11. Thestacked semiconductor device according to claim 9, wherein the first dieincludes a recess proximate the first surface coupling region.
 12. Amethod comprising: placing a screen mask on surfaces of a plurality ofsemiconductor dies, each of the semiconductor die includes a raisedportion and a recessed portion, wherein placing the screen mask on thesurfaces comprises placing the screen mask directly on a surface of theraised portion without touching a surface of the recessed portion;dispensing a paste on the screen mask; causing the paste to pass throughapertures of the screen mask that are located over the raised portion,without causing the paste to pass through apertures of the screen maskthat are located over the recessed portion; and removing the screenmask, wherein a layer of paste remains on the surface of the raisedportion of the semiconductor dies when the screen mask is removed. 13.The method according to claim 12, wherein the paste is an epoxy resin.14. The method according to claim 12, wherein causing the paste to passthrough the apertures of the screen mask that are located over theraised portion, without causing the paste to pass through apertures ofthe screen mask that are located over the recessed portion comprisesapplying pressure that presses the paste though the apertures.
 15. Themethod according to claim 12, wherein the semiconductor dies are firstsemiconductor dies, the method further comprising securing secondsemiconductor dies to the first semiconductor dies, respectively, usingthe paste.
 16. The method according to claim 15, wherein the firstsemiconductor dies integrates a MEMS device and the second semiconductordies integrates an ASIC.